Permanent magnet motor

ABSTRACT

To provide a highly efficient permanent magnet motor which can reduce armature reaction flux, improve magnetic flux distribution in an outer core, and thereby reduce noise and vibration.  
     A permanent magnet motor has permanent-magnet-holding slots  5  formed in those parts of a rotor core  2 A which correspond to sides of an approximately regular polygon centered on an axis of the rotor core  2 A, permanent magnets  4  inserted in the respective permanent-magnet-holding slots, and four or more radially elongated slits  6  arranged apart from each other along each of the permanent-magnet-holding slots on an outer core outside the permanent-magnet-holding slots, characterized in that at a radially outer end, the slits are spaced approximately equally while at a radially inner end, spacing between the slits is reduced with increasing distance from a center of each permanent magnet, with the spacing at the center being the largest.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a permanent magnet motor equipped witha rotor consisting of a plurality of permanent magnets buried in a rotorcore.

2. Related Art

There is disclosed a permanent magnet motor whose demagnetizationresistance and efficiency has been improved by burying permanent magnetsin a rotor core (see, for example, Japanese Patent Laid-Open No.11-187597). FIG. 11 is a side view showing an end of a rotor of thepermanent magnet motor as viewed along the insertion direction of arotating shaft of the rotor before the rotating shaft is inserted. InFIG. 11, a rotor 2 consists of a rotor core 2 a and rotating shaft (notshown), where the rotor core 2 a is a generally pillar-shaped stack ofsteel plates cylindrical in outline. Near its outer circumference, therotor core 2 a has permanent-magnet-holding slots 5 corresponding tosides of an approximately regular octagon and each of thepermanent-magnet-holding slots 5 contains a permanent magnet 4. Thepermanent magnets 4 are arranged in such a way that the S pole and Npole alternate with each other. A plurality of radially elongated slits6 are arranged apart from each other along each of thepermanent-magnet-holding slots 5 on an outer core 3 outside thepermanent-magnet-holding slots 5. A rotating-shaft hole 8 is provided inthe center of the rotor core 2 a to accept the rotating shaft.

With the conventional permanent magnet motor described above, the slits6 in the rotor core 2 a are spaced at equal intervals to lead in and outmagnetic flux of the permanent magnets 4 radially as well as to preventmagnetic flux (hereinafter referred to as armature reaction flux)generated by stator winding current from being bent along thecircumference of the outer core 3.

However, when the plurality of slits 6 are arranged at equal intervals,radial magnetic flux distribution in the permanent magnets 4 has atrapezoidal profile. Consequently, geometric positional relationshipbetween stator and rotor produces high cogging torque, increasingvibration.

Also, voltage induced by stator windings has a harmonic-rich distortedwaveform, resulting in not only increased noise, but also increased coreloss, which in turn results in reduced efficiency.

Furthermore, if the stator is driven by 3-phase sine wave AC current,only a fundamental wave component contribute effectively to torque whileharmonic components produce torque ripple which increases vibration andnoise.

SUMMARY OF THE INVENTION

In order to solve the above-described problems, an object of the presentinvention is to provide a highly efficient permanent magnet motor whichcan reduce armature reaction flux, improve magnetic flux distribution inan outer core, and thereby reduce noise and vibration.

To achieve the above object, the present invention provides a permanentmagnet motor set forth in claims 1 to 5.

Claim 1 of the present invention sets forth a permanent magnet motorcomprising a rotor equipped with a rotor core which is a generallypillar-shaped stack of steel plates, permanent-magnet-holding slotsformed in those parts of the rotor core which correspond to sides of anapproximately regular polygon centered on an axis of the rotor core,permanent magnets inserted in the respective permanent-magnet-holdingslots, and a plurality of radially elongated slits arranged apart fromeach other along each of the permanent-magnet-holding slots on an outercore outside the permanent-magnet-holding slots, characterized in thatat a radially outer end, the slits are spaced approximately equallywhile at a radially inner end, spacing between the slits is reduced withincreasing distance from a center of each permanent magnet, with thespacing at the center being the largest.

Claim 2 of the present invention sets forth the permanent magnet motoraccording to claim 1, characterized in that if sides of the permanentmagnets are made to correspond to a base of a sine wave, the spacingbetween the slits at the radially inner end is proportional to height ofthe sine wave.

Claim 3 of the present invention sets forth the permanent magnet motoraccording to claim 2, characterized in that the rotor has 2 n magneticpoles and a stator has 3 n teeth each of which has a conductor wound ina concentrated manner, where n is a positive integer; and sides of thepermanent magnets correspond to the base of the sine wave whencontracted toward the center.

Claim 4 of the present invention sets forth the permanent magnet motoraccording to claim 1, characterized in that core width between aradially outer end of the slits and an outer circumference of the rotorcore is larger at the center of the permanent magnets than at both ends.

Claim 5 of the present invention sets forth the permanent magnet motoraccording to claim 1, characterized in that core width between aradially outer end of the permanent-magnet-holding slots and a radiallyinner end of the slits as well as core width between a radially innerend of the slits and an outer circumference of the rotor core are 1 to 3times thickness of the steel plates.

The present invention reduces armature reaction flux, improves magneticflux distribution in an outer core, and thus provides a highly efficientpermanent magnet with reduced noise and vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing an end of a rotor of a permanent magnetmotor according to a first embodiment of the present invention as viewedalong the insertion direction of a rotating shaft before the rotatingshaft is inserted;

FIG. 2 is a side view showing an end of a rotor of a permanent magnetmotor according to a second embodiment of the present invention asviewed along the insertion direction of a rotating shaft before therotating shaft is inserted;

FIG. 3 is a partially enlarged view illustrating, in detail, spacing atthe radially inner end of slits formed in an outer core of the rotorcore shown in FIG. 2;

FIG. 4 is a waveform chart used to determine the spacing at the radiallyinner end of the slits formed in the outer core of the rotor core shownin FIG. 2;

FIG. 5 is a side view showing an end of a rotor and stator of apermanent magnet motor according to a third embodiment of the presentinvention;

FIG. 6 is a waveform chart used to determine the spacing at the radiallyinner end of slits formed in an outer core of the rotor core shown inFIG. 5;

FIG. 7 is a distribution chart of magnetic flux generated in the rotorcore shown in FIG. 5;

FIG. 8 is a partially enlarged side view showing an end of a rotor of apermanent magnet motor according to a fourth embodiment of the presentinvention as viewed along the insertion direction of a rotating shaft;

FIG. 9 is an induced voltage waveform chart illustrating operation ofthe fourth embodiment shown in FIG. 8;

FIG. 10 is a diagram showing a drive circuit which drives the permanentmagnet motor of each embodiment; and

FIG. 11 is a side view showing an end of a rotor of a permanent magnetmotor as viewed along the insertion direction of a rotating shaft of therotor before the rotating shaft is inserted.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, the present invention will be described more specificallywith reference to embodiments shown in the drawings.

FIG. 1 is a side view showing an end of a rotor of a permanent magnetmotor according to a first embodiment of the present invention as viewedalong the insertion direction of a rotating shaft before the rotatingshaft is inserted. In the figure, the same components as theconventional motor in FIG. 11 are denoted by the same reference numeralsas the corresponding components in FIG. 11, and description thereof willbe omitted. A rotor 2 consists of a rotor core 2A and a rotating shaft(not shown), where the rotor core 2A is a generally pillar-shaped stackof steel plates cylindrical in outline.

The rotor core 2A has permanent-magnet-holding slots 5 formed atlocations corresponding to sides of an approximately regular quadranglenear the outer circumference of the rotor core 2A. A permanent magnet isburied in each of the permanent-magnet-holding slots 5. A plurality ofradially elongated slits 6, for example, ten slits, are arranged apartfrom each other along each of the permanent-magnet-holding slots 5 on anouter core 3 outside the permanent-magnet-holding slots 5. Arotating-shaft hole 8 is provided in the center of the rotor core 2A toaccept a rotating shaft and through-bolt holes 9 are provided around therotating-shaft hole 8.

According to this embodiment, at the radially outer end, the slits 6 arespaced approximately equally along the outer circumference of the rotorcore 2A while at the radially inner end, spacing between the slits isreduced with increasing distance from the center of each permanentmagnet 4, with the spacing at the center of the permanent magnet 4 beingthe largest.

Since the slits 6 are arranged as described above, magnetic flux of thepermanent magnets 4 passing through pole cores between the slits 6 isapproximately proportional to the spacing at that end of the slits 6which is nearer to the permanent magnets 4. Consequently, distributionof the permanent magnets' (4) magnetic flux passing through the outercore 3 is reduced with increasing distance from the center of eachpermanent magnet 4, with the spacing at the center of the permanentmagnet 4 being the largest when viewed in the circumferential direction.This increases effective magnetic flux which contributes to torquegeneration, produces less harmonic magnetic flux and core loss than inthe case of trapezoidal magnetic flux distribution, and reduces coggingtorque which causes vibration.

Thus, the first embodiment reduces armature reaction flux using theplurality of slits, improves the magnetic flux distribution in the outercore, and thereby provides a highly efficient permanent magnet withreduced noise and vibration.

FIG. 2 is a side view showing an end of a rotor of a permanent magnetmotor according to a second embodiment of the present invention asviewed along the insertion direction of a rotating shaft before therotating shaft is inserted. In the figure, the same components as thefirst embodiment in FIG. 1 are denoted by the same reference numerals asthe corresponding components in FIG. 1, and description thereof will beomitted. A rotor core 2B has twelve slits 6 arranged apart from eachother along each of the permanent-magnet-holding slots 5. At theradially outer end, the slits 6 are spaced approximately equally alongthe outer circumference of the rotor core 2B. At the radially inner end,a side of each permanent magnet is made to correspond to the base of asine wave and the slits are arranged such that the spacing between themwill be proportional to the height of the sine wave.

FIG. 3 is a partially enlarged view illustrating, in detail, spacing atthe radially inner end of slits 6 formed in the outer core 3 outside thepermanent magnet 4 in the rotor core 2B. The spacing at the longitudinalcenter of the permanent magnet 4 is denoted by P1 and spacings P2, P3, .. . P6, and P7 are determined in such a way that they will be reducedwith increasing distance from the center. As shown in FIG. 4, the baseof a sine wave (half wave) equal to 180 degrees in electrical angle isbrought into correspondence with the length W of the permanent magnet 4,the sine wave is divided into 15 parts along the electrical angle, theheight of the center part is denoted by P1 and the heights of the outerparts are denoted by P2, P3, . . . P6, and P7 in sequence, and thespacings P1 to P7 at the radially inner end of the slits 6 aredetermined in such a way as to be proportional to the heights P1 to P7.Thus, a relationship P1>P2>P3>P4>P5>P6>P7 holds true. Also, the magneticflux is proportional to the spacings P1 to P7, resulting in a magneticflux distribution close to the sine wave.

Thus, the second embodiment reduces armature reaction flux using theplurality of slits, further improves the magnetic flux distribution inthe outer core, and thereby provides a highly efficient permanent magnetwith reduced noise and vibration.

FIG. 5 is a side view showing an end of a rotor and stator of apermanent magnet motor according to a third embodiment of the presentinvention. In particular, it employs, as the rotor 2, a rotor core 2Csimilar to the rotor core 2B shown in FIG. 3. The spacings P1 to P7 atthe radially inner end of the slits 6 are determined here assuming thatthe rotor 2 has 2 n (=4) magnetic poles while a stator 1 has 3 n (=6)teeth each of which has a conductor 7 wound in a concentrated manner,where n is a positive integer (2 in this case) and assuming that sidesof the permanent magnets 4, when contracted toward the center,correspond to the base of the sine wave shown in FIG. 4.

If there is a relationship of 2 n:3 n between the number of rotor (2)poles and number of stator (1) teeth, angular intervals between thepoles of the rotor 2 are smaller than angular intervals between theteeth of the stator 1, and thus the teeth in each phase of the stator 1cannot receive all the magnetic flux from one pole of the rotor 2.

FIG. 6 is an explanatory diagram illustrating how to solve this problem.The rotor core 2C is configured as follows: instead of putting theentire length W of the permanent magnet 4 in correspondence with thebase of a sine wave directly, a sine wave with the same area as the sinewave shown in FIG. 4 is created in a segment obtained by contracting thepermanent magnet 4 inward by a predetermined length from its oppositeends, and spacings P1 to P7 at the radially inner end of the slits 6 aredetermined in such a way as to be proportional to heights P1 to P7obtained by equally dividing the created sine wave. Consequently, asshown in FIG. 7, compared to the magnetic flux distribution according tothe second embodiment represented by a broken curve P, the magnetic fluxdistribution according to the third embodiment is closer to the sinewave, rising sharply and intensifying in the midsection as indicated bya solid curve Q. Thus, the third embodiment provides a better inducedvoltage waveform than the second embodiment, and further improves themotor efficiency.

Thus, the third embodiment reduces armature reaction flux using theplurality of slits, further improves the magnetic flux distribution inthe outer core, and thereby provides a highly efficient permanent magnetmotor with reduced noise and vibration.

FIG. 8 is a partially enlarged side view showing an end of a rotor of apermanent magnet motor according to a fourth embodiment of the presentinvention as viewed along the insertion direction of a rotating shaft.In the figure, the same components as in FIG. 3 are denoted by the samereference numerals as the corresponding components in FIG. 3, anddescription thereof will be omitted. In a rotor core 2D, core widthbetween the radially outer end of the slits 6 and outer circumference ofthe rotor core 2D is larger (e.g., d2) at the center of the permanentmagnet than at locations distant from the center (e.g., d1). The reasonwill be explained below.

If the core width between the radially outer end of the slits 6 and theouter circumference of the rotor core were uniform, rippling, i.e.,pulsation with a small amplitude, might occur in the midsection of theinduced voltage waveform where the magnetic flux is intense, as shown inFIG. 9. Maybe this is because the core width between the radially outerend of the slits 6 and the outer circumference of the rotor core is toosmall for the intensity of the magnetic flux, resulting in high magneticreluctance. By making the core width between the radially outer end ofthe slits 6 and the outer circumference of the rotor core 2D larger (d2)near the center of the pole where rippling occurs prominently in themagnetic flux distribution than in the other part (d1), the rotor core2D according to this embodiment prevents rippling in the induced voltagewaveform without ruining the effect of magnetic flux distributioncontrol performed by means of the slits 6.

Thus, the fourth embodiment reduces armature reaction flux using theplurality of slits, further improves the magnetic flux distribution inthe outer core, and thereby provides a highly efficient permanent magnetmotor with reduced noise and vibration.

FIG. 10 shows a drive circuit which drives the permanent magnet motor.An alternating current from an AC power supply 10 is converted into adirect current by a converter 11. The DC output is reconverted into analternating current of a desired frequency by an inverter 12 capable ofproducing variable-frequency output and the resulting alternatingcurrent is supplied to a permanent magnet motor 13 (hereinafter referredto as the motor 13). A position sensor 14 detects rotor position of themotor 13 based on terminal voltage of the motor 13 and sends it as aposition signal to a controller 15. Using the position signal, thecontroller 15 controls the inverter 12 in such a way that outputfrequency of inverter 12 will be synchronized with rotational frequencyof the motor 13. Incidentally, the motor 13 is practically a 3-phasesynchronous motor and the inverter 12 is configured to be a 3-phasebridge type accordingly. U, V, and W-phase positive arms are denoted byU+, V+, and W+, respectively, while negative arms are denoted by U−, V−,and W−. The illustrated circuit configuration constitutes a DCcommutatorless motor. Incidentally, the method for detecting the rotorposition of a motor based on the terminal voltage of the motor is knownas a sensorless method or indirect position detection method.

As is well known, in each phase winding of the motor 13, a pair of apositive arm of one phase and a negative arm of another phase turn onand the other arms remain off at each instant. As the arm to be turnedon is switched among the three phases in sequence, a desired 3-phaseoutput is given to the motor 13 from the inverter 12. This makes itpossible to drive the stator by supplying 3-phase sine wave AC current.

It is desirable to minimize the core width between the radially outerend of the permanent-magnet-holding slots 5 and radially inner end ofthe slits 6 as well as the core width between the radially outer end ofthe slits 6 and an outer circumference of the rotor core from theviewpoint of preventing the magnetic flux from bending along thecircumference. On the other hand, it is desirable to provide someallowance when blanking the steel plates. To reduce the rippling,appropriately, the core widths are 1 to 3 times the thickness of themagnetic steel plate.

Although in the above embodiments, permanent magnets are buried in thoseparts of the rotor core which correspond to the sides of a regularquadrangle centered on the axis of the rotor core, the present inventionis not limited to this. The present invention is applicable to almostany permanent magnet motor whose rotor consists of permanent magnetsburied in those parts of the rotor core which correspond to the sides ofan approximately regular polygon centered on the axis of the rotor core.

Furthermore, although in the above embodiments, ten to twelve slits 6are formed in the outer core 3, the present invention is applicable toany permanent magnet motor which has four or more slits 6, consideringthat slits are arranged in relation to a sine wave.

1. A permanent magnet motor comprising a rotor equipped with a rotorcore which is a generally pillar-shaped stack of steel plates,permanent-magnet-holding slots formed in those parts of the rotor corewhich correspond to sides of an approximately regular polygon centeredon an axis of the rotor core, permanent magnets inserted in therespective permanent-magnet-holding slots, and a plurality of radiallyelongated slits arranged apart from each other along each of thepermanent-magnet-holding slots on an outer core outside thepermanent-magnet-holding slots, characterized in that at a radiallyouter end, the slits are spaced approximately equally while at aradially inner end, spacing between the slits is reduced with increasingdistance from a center of each permanent magnet, with the spacing at thecenter being the largest.
 2. The permanent magnet motor according toclaim 1, characterized in that if sides of the permanent magnets aremade to correspond to a base of a sine wave, the spacing between theslits at the radially inner end is proportional to height of the sinewave.
 3. The permanent magnet motor according to claim 2, characterizedin that the rotor has 2 n magnetic poles and a stator has 3 n teeth eachof which has a conductor wound in a concentrated manner, where n is apositive integer; and sides of the permanent magnets correspond to thebase of the sine wave when contracted toward the center.
 4. Thepermanent magnet motor according to claim 1, characterized in that corewidth between a radially outer end of the slits and an outercircumference of the rotor core is larger at the center of the permanentmagnets than at both ends.
 5. The permanent magnet motor according toclaim 1, characterized in that core width between a radially outer endof the permanent-magnet-holding slots and a radially inner end of theslits as well as core width between a radially outer end of the slitsand an outer circumference of the rotor core are 1 to 3 times thicknessof the steel plates.
 6. The permanent magnet motor according to claim 1,characterized in that at least four slits are arranged along each of thepermanent magnet slots.